An electric power steering apparatus for a vehicle that detects a steering torque applied to a steering wheel which is turned by a driver (hereinafter, referred to as a “driver's steering wheel”) and that causes an electric motor to produce an assist torque based on the detected steering torque has been used. An in-vehicle battery is used as an electric power supply unit for the electric power steering apparatus. There is also an electric power steering apparatus that is provided with a plurality of batteries in order to allow the electric power steering apparatus to function properly even if a malfunction occurs in one of the batteries. For example, Japanese Patent Application Publication No. JP-2000-236626 (JP-A-2000-236626) describes an electric power steering apparatus that is provided with a high-tension battery and a low-tension 12-V battery as electric power supply units used to drive an electric motor of the electric power steering apparatus. When electric power supply from the high-tension battery is shut off, electric power is supplied to the electric motor from the low-tension battery that serves as a back-up electric power supply unit.
However, when electric power is supplied to the electric motor from the low-tension battery, the electric motor does not rotate at a high speed. The required torque (assist torque) that needs to be produced by the electric motor is set based on the turning operation of the driver's steering wheel. As shown in FIG. 12, the motor speed ω2 at which the required torque is produced when the low-tension battery is used is considerably lower than the motor speed ω1 at which the required torque is produced when the high-tension battery is used. If a driver turns the driver's steering wheel gradually, no inconvenience is caused. However, if the driver turns the driver's steering wheel quickly, the motor speed does not respond accurately to the speed at which the driver turns the driver's steering wheel, which imposes resistance to the turning operation of the driver's steering wheel. The electric motor is connected to a steering mechanism that steers steering wheels of the vehicle. Accordingly, when the electric motor is not allowed to rotate at a high speed due to the usage of the low-tension battery, if the driver's steering wheel is turned quickly, the steering wheels are steered only by the steering force applied to the driver's steering wheel by the driver. Namely, the electric power steering apparatus does not appropriately assist the turning operation of the driver's steering wheel.
Such an inconvenience is minimized, for example, by providing a voltage-increasing circuit on the output side of the low-tension battery. However, provision of such voltage-increasing circuit causes other inconveniences such as a cost increase and necessity to create space for the voltage-increasing circuit.